In recent years, as an image forming apparatus configured to form an image on a recording medium, there is an apparatus configured to erase an image on a sheet and reuse the sheet. The reuse of the sheet saves paper resources and realizes environmental protection. Further, as the image forming apparatus, there is an apparatus configured to perform, in order to realize low cost and space saving, a process for forming an image with an erasable toner and a process for forming an image with a normal toner. For example, JP-A-06-95494 discloses an apparatus configured to select the normal toner or the erasable toner and perform an image forming process.
However, even if the image on the sheet is erased, ingredients of the erasable ink or toner remain on the sheet. Therefore, when the image formation and the image erasing on the sheet are repeated, an area where the ingredients of the erased sheet or toner remain on the sheet expands. In the area where the ingredients of the ink or toner remain, it is likely that the ink less easily penetrates the sheet during the next image formation and image quality is deteriorated. For example, when the image on the sheet is erased by heat, it is likely that, while the image formation and image erasing on the sheet are repeated, characteristics of the sheet are changed by the influence of the heat and image quality is deteriorated.
Therefore, there is a demand for development of an image forming apparatus configured to grasp a state of a sheet and control an image forming process according to the state of the sheet.